The present invention pertains to forming rolls and to the connection of such forming rolls to supporting arbors. It especially pertains to rolls for rod rolling and their mounting on cantilever type tapered rolling mill arbors.
Rod rolling mills consist of a frame supporting a pair of driven arbors on which are mounted rolls. Each roll has a working surface in which one or more contoured grooves have been placed. The material being worked is passed through the gap formed by the grooves in a pair of rolls and thereby obtains the desired cross sectional configuration and/or reduction in size.
The conditions at the roll work surface can be very abrasive. Cemented carbide rolls, because of their high wear resistance, have, therefore, replaced steel rolls in many rod rolling applications. The high pressures produced when rolling rod also favor the use of cemented carbide because of its high compressive strength.
However, cemented carbide forming rolls present mounting problems, in that the rolls are preferably mounted without a substantial amount of tensile stress being developed therein. One mounting mode preloads the rolls in compression, at least in a direction parallel to their axis of rotation.
Examples of such roll mounting arrangements are shown in U.S. Pat. No. 3,786,546, assigned to applicant corporation; in applicant corporation's brochure entitled "Designing with Kennametal;" U.S. Pat. No. 2,342,159 and British Patent Specification No. 1,420,693. In these arrangements, the majority of the torque from the driven arbor is transmitted to the roll through cylindrical metallic rings attached to the arbor and clamped against both axial faces of the roll.
The problem associated with this mounting arrangement is that the bore of the carbide roll and the mating surface on the arbor should be machined to very tight tolerances in order to assure that the carbide roll will be radially supported all around the circumference of its bore by the arbor. Failure to achieve this desired fit may lead to early roll failure.
In an alternative mounting arrangement, a tapered cylindrical wedge member or sleeve is pressed in between the tapered arbor and the carbide roll bore with sufficient force such that the static frictional forces between the arbor, wedge and roll are sufficient to transmit the torque from the arbor. Examples of this mounting arrangement are described in Iron and Steel Engineer, April 1975, Pages 80 to 88. This mounting arrangement, however, produces tensile stresses in the carbide roll near its bore. In order to assure that these tensie stresses will not lead to a failure of the roll, these rolls are designed with increased wall thickness, increased width, or increased width with a keyway to assist in transmitting torque loads.
Keyways in cemented carbide rolls are undesirable since they act as stress raisers. Increasing the width or wall thickness of the roll is also undesirable since it increases the amount of cemented carbide used, thereby increasing the cost of the roll. The metals used in these rolls, for example, Co, W, Ti and Ta, are typically very expensive and of limited availability. Raw material costs, therefore, make up a significant portion of roll manufacturing costs.
In an alternate embodiment of the mounting system just described, the cylindrical wedge member is slotted. This reduces the tensile forces in the carbide roll but does not provide sufficient frictional force to hold the roll nonrotational on the arbor. Keyways are formed in these rolls and they are keyed to the arbor to assure adequate torque transmission. However, as already noted. keyways act as stress concentrators and can, therefore lead to early roll failure. They also increase the cost of the roll, since it is required that the cemented carbide roll have an increased width to accommodate the keyway. Mounting systems utilizing these concepts are illustrated in U.S. Pat. Nos. 3,514,136; 3,514,137; and 3,727,957; all assigned to applicant corporation.